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Which handlers run?

importance: 5

There’s a button in the variable. There are no handlers on it.

Which handlers run on click after the following code? Which alerts show up?

button.addEventListener("click", () => alert("1"));

button.removeEventListener("click", () => alert("1"));

button.onclick = () => alert(2);

The answer: 1 and 2.

The first handler triggers, because it’s not removed by removeEventListener. To remove the handler we need to pass exactly the function that was assigned. And in the code a new function is passed, that looks the same, but is still another function.

To remove a function object, we need to store a reference to it, like this:

function handler() {
  alert(1);
}

button.addEventListener("click", handler);
button.removeEventListener("click", handler);

The handler button.onclick works independantly and in addition to addEventListener.